The present application is based upon Japanese Patent Application No. 2000-371471 filed Dec. 6, 2000 and Japanese Patent Application No. 2001-132746 filed Apr. 27, 2001, the entire disclosures of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a wavelength monitor and a semiconductor laser device in which a wavelength of a laser beam outputted from a semiconductor laser is monitored.
2. Description of Related Art
A dense wavelength division multiplexing (DWDM) optical communication has been performed in an optical communication field using optical fibers. In this DWDM optical communication, laser beams, which are emitted from a number of semiconductor lasers and have various wavelengths, pass through a plurality of optical fibers and are multiplexed to produce a multiplexed laser beam, the multiplexed laser beam is lead to an optical fiber, and the multiplexed laser beam is transmitted to a destination. Thereafter, the multiplexed laser beam is demultiplexed to a plurality of laser beams, and the laser beams are used for various purposes.
In recent years, a technique in which laser beams are multiplexed at high density has been proposed in order to efficiently transmit the laser beams. In this technique, wavelength intervals of the laser beams to be multiplexed are narrowed (for example, wavelength intervals of the laser beams are set to specific wavelength intervals equivalent to 50 GHz). Therefore, to multiplex the laser beams without interfering with each other, it is required for each semiconductor laser device to set a wavelength of the laser beam with high stability. To achieve this requirement, an intensity and wavelength of a backward-directed laser beam (also referred to as a backward laser beam), which is emitted from a semiconductor laser simultaneously with a forward-directed laser beam (also referred to as a forward laser beam) to an optical fiber, is detected and monitored, and the wavelength of the backward laser beam is controlled according to the intensity of the backward laser beam to adjust an wavelength of the forward laser beam. Also, in a laser beam measuring field, an intensity and wavelength of a backward laser beam emitted from a semiconductor laser is monitored, and a wavelength of a beam of homogeneous light emitted from the semiconductor laser is measured with high precision.
FIG. 23 is a view schematically showing the configuration of a conventional wavelength monitor in which an intensity and varying wavelength of a backward laser beam emitted from a semiconductor laser is monitored. The conventional wavelength monitor shown in FIG. 23 is disclosed in Published Unexamined Japanese Patent Application H10-79551 (1998). As shown in FIG. 23, a backward laser beam emitted from a semiconductor laser 126 is collimated in a lens 127, and the collimated laser beam transmits through a quarter wavelength plate 128 to transform a linear polarization of the laser beam into a circular polarization. Thereafter, the circularly polarized laser beam is incident on a first polarized beam splitter 129 to divide the incident laser beam into a first laser beam 130 and a second laser beam 131. The first polarized beam splitter 129 has a band pass filter film 132 placed on a first output end face. The first laser beam 130 transmits through the band pass filter film 132 and is received in a first photodiode 133. An output of electric current of the first laser beam 130 detected in the first photodiode 133 fluctuates with a varying wavelength of the backward laser beam emitted from the semiconductor laser 126. The second laser beam 131 is incident on a second polarized beam splitter 134 to divide the incident laser beam into a third laser beam 135 and a fourth laser beam 136. The second polarized beam splitter 134 has a band pass filter film 137 placed on a third output end face. The third laser beam 135 transmits through a band pass filter film 137 and is received in a second photodiode 138. An output of electric current of the third laser beam 135 detected in the second photodiode 138 fluctuates with a varying wavelength of the backward laser beam emitted from the semiconductor laser 126. The fourth laser beam 136 is received in a third photodiode 139 to detect an output of electric current of the fourth laser beam 136. In the conventional wavelength monitor, the outputs of electric current detected in both the first photodiode 133 and the second photodiode 138 are used to monitor the wavelength of the backward laser beam emitted from the semiconductor laser 126, and the output of electric current detected in the third photodiode 139 is used to monitor the intensity of the backward laser beam emitted from the semiconductor laser 126. Therefore, the wavelength and intensity of a forward laser beam emitted from the semiconductor laser 126 can be stabilized.
However, because the conventional wavelength monitor has the above-described configuration, two polarized beam splitters 129 and 134 and two band pass filters 132 and 137 are required. Therefore, a problem has arisen that the number of parts is increased in the conventional wavelength monitor so as to heighten a product cost.
Also, because the backward laser beam emitted from the semiconductor laser 126 is split to propagate in three directions, optical elements such as the lens 127 adjusting the convergence of the backward laser beam emitted from the semiconductor laser 126, the polarized beam splitters 129 and 134 and the photodiodes 133, 138 and 139 are widely separated in a plane. In this case, another problem has arisen that it is difficult to accurately arrange the optical elements with respect to the backward laser beam propagated in three directions.
Also, because three plates, on which the photodiodes 133, 138 and 139 are arranged respectively, separately move in different directions due to a temperature variation and/or a mechanical variation occurring over a long period of time, a positional relationship among the semiconductor laser 126, the lens 127 and the photodiodes 133, 138 and 139 undergoes variation. In this case, another problem has arisen that an intensity of the laser beam detected in each photodiode fluctuates even though an intensity of the laser beam emitted from the semiconductor laser 126 is constant.
Also, because the second and third photodiodes 138 and 139 are arranged on two planes positioned orthogonal to each other, the planes separately move in different directions due to a temperature variation and/or a mechanical variation occurring over a long period of time. Therefore, another problem has arisen that an output of electric current of the laser beam detected in each photodiode is not stabilized.
In Published Unexamined Japanese Patent Application H5-149793 (1993), a conventional semiconductor laser device is disclosed. In this device, a semiconductor laser and a wavelength monitor for detecting a varying wavelength of a laser beam emitted from the semiconductor laser are arranged. Also, in Published Unexamined Japanese Patent Application S58-12831 (1983), a wavelength measuring device for detecting a wavelength of a laser beam is disclosed. In these devices, a laser beam emitted from a beam source is directly received by one photodetector. Also, the laser beam is received by another photodetector through a filter. These photodetectors are placed on a carrier. In this case, though the precision of the position of the photodetectors is relatively high with respect to a vertical direction, it is difficult to precisely arrange the photodetectors in horizontal directions. Therefore, a problem has arisen that it is difficult to precisely arrange the photodetectors in horizontal directions such that the whole laser beam is correctly detected in the photodetectors or such that a preset part of the laser beam is correctly detected in each photodetector. Also, positions of the photodetectors shift from each other in horizontal directions due to a temperature variation and/or a mechanical variation occurring over a long period of time. Therefore, another problem has arisen that it is difficult to stably and correctly detect a wavelength of the laser beam.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional wavelength monitor and the conventional semiconductor laser device, a wavelength monitor and a semiconductor laser device in which a wavelength of a laser beam emitted from a semiconductor laser is always monitored with high precision.
Also, another object of the present invention is to provide a wavelength monitor and a semiconductor laser device in which an intensity and varying wavelength of a laser beam emitted from a semiconductor laser is correctly monitored by using a simple arrangement of constituent elements that does not require a plurality of polarized beam splitters and a plurality of band pass filters.
According to one aspect of the present invention, a wavelength monitor is provided comprising a cylindrical lens configured to allow a laser beam emitted from a semiconductor laser to pass therethrough, first and second photodetectors configured to receive the laser beam passed through the cylindrical lens, and a wavelength filter disposed in an optical path between the semiconductor laser and the first photodetector.
According to another aspect of the present invention, a semiconductor laser device is provided comprising a semiconductor laser configured to emit a laser beam, a cylindrical lens configured to allow a laser beam emitted from a semiconductor laser to pass therethrough, first and second photodetectors configured to receive the laser beam passed through the cylindrical lens, and a wavelength filter disposed in an optical path between the semiconductor laser and the first photodetector.
In the above configurations, the laser beam transmitted through the cylindrical lens is uniaxially converged in a convergence direction, and the uniaxially converged laser beam is received by the first and second photodetectors. Therefore, even if the position of an optical element such as the semiconductor laser, the first photodetector or the second photodetector is shifted in the convergence direction, the laser beam is still received by the first and second photodetectors. Also, even if the position of an optical element is shifted in a direction perpendicular to the convergence direction in a plane of beam receiving faces of the first and second photodetectors, a beam area of the laser beam received by each photodetector does not change.
Accordingly, the wavelength of the laser beam emitted from the semiconductor laser can be always monitored with high precision according to both portions of the laser beam received by the first and second photodetectors, respectively, regardless of whether the position of the semiconductor laser, the first photodetector or the second photodetector is shifted.